WLAN having load balancing based on access point loading

ABSTRACT

A network having distribution of access point loading includes access points to which mobile stations can associate themselves based upon access point beacon signal levels and loading levels for the various access points. A mobile station receives beacon signals from various access points and determines a signal strength for the received beacon signals. The mobile station also receives access point loading information from the access points. The mobile station associates with an access point based upon the access point beacon signal strengths and the access point loading information.

CROSS REFERENCE TO RELATED APPLICATIONS

The present invention claims the benefit of U.S. Provisional PatentApplication No. 60/332,957, filed on Nov. 19, 2001, which isincorporated herein by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

Not Applicable.

FIELD OF THE INVENTION

The present invention relates generally to communication networks and,more particularly, to wireless communication networks.

BACKGROUND OF THE INVENTION

Wireless communication networks for providing links to mobile stationsare well known in the art. In one type of wireless network, a series ofaccess points provide wireless connections to various mobile users. Forexample, a building can include access points located at strategiclocations to serve mobile users as they move throughout the building.The mobile users migrate from access point to access point based uponthe strength of beacon signals from the various access points. That is,the mobile stations use the strength of the beacon signals to select thebest access point at a given point in time.

With changes in the channel environment and number of users in aWireless Local Area Network (WLAN) system, different access pointsexperience different traffic loading. That is, the number of usersserved by each of the access points varies over time. Those accesspoints that serve a relatively high number of stations (hot spots) canbecome overloaded and experience reduced performance. For example, anaccess point can become overloaded during a meeting in a conference roomproximate the access point when the attendees attempt to connect theirlaptops to the corporate intranet.

It would, therefore, be desirable to adjust the loading of networkaccess points to reduce network congestion.

SUMMARY OF THE INVENTION

The present invention provides a wireless network having mobile stationsthat determine to which access point they will associate based uponreceived beacon signal power levels and access point load informationsent to the mobile station. With this arrangement, overall networkperformance is enhanced by more efficient access point loading. Whilethe invention is primarily shown and described in conjunction withwireless access points having beacons, it is understood that theinvention is applicable to wireless networks in general in which it isdesirable to distribute loading.

In one aspect of the invention, network wireless access points transmitbeacon signals to mobile stations within their coverage area and themobile stations determine the strength of each received beacon signal.Based upon the beacon signal strength, a particular mobile station candetermine which access points the mobile station can associate with. Theaccess points also broadcast, such as in a Beacon frame, loading levelinformation for the access point. The mobile station then selects anaccess point based upon beacon signal strength and access point loading.By taking into consideration access point loading levels, networkcongestion due to overloaded access points can be reduced.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be more fully understood from the following detaileddescription taken in conjunction with the accompanying drawings, inwhich:

FIG. 1 is a schematic depiction of a wireless network having accesspoint selection by mobile stations in accordance with the presentinvention;

FIG. 2 is a schematic block diagram of an exemplary access point thatcan form a part of the network of FIG. 1;

FIG. 3 is a pictorial representation of an exemplary capability fieldthat can be contained in a message exchanged by an access point and amobile station in accordance with the present invention; and

FIG. 4 is a schematic depiction of a mobile station that can form a partof the network of FIG. 1.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows an exemplary wireless network 100 having access point loadbalancing in accordance with the present invention. The network 100includes a series of access points AP_(a-N) having associated coverageareas CA_(a-P) serving the various mobile stations MSa-M. Access pointload balancing is achieved by each of the mobile stations MS_(a-M)determining to which one of the access points AP_(a-N) the mobilestation should associate with based upon access point beacon signalpower levels and access point loading information, as described morefully below.

In general, each mobile station MS receives a beacon signal from accesspoints Ap_(a-N) having a coverage area CA_(a-P) in which the mobilestation is located. The access points AP also broadcast, such as in aso-called Beacon Frame, loading information for the access point, asdescribed further below. Mobile stations Ms_(a-M) that receive multipleaccess point beacon signals above a predetermined threshold level, forexample, can select one of these access points based upon the load levelof the access points to optimize overall access point loading. Bydistributing mobile station associations based on access point loading,network congestion is reduced and overall network performance isenhanced.

For example, as illustrated in FIG. 1, a first mobile station MS_(c) maybe located within the coverage areas CA_(a,b) of associated first andsecond access points AP_(a,b). The first mobile station MS_(c) receivesa beacon signal of acceptable power level, e.g., above a predeterminedthreshold, from the first and second access points AP_(a,b). In anexemplary embodiment, the first and second access points AP_(a,b) alsobroadcast their loading levels in a Beacon Frame that is received by thefirst mobile station MS_(c). Since the first access point AP_(a)supports significantly more mobile stations, e.g., MS_(d-i), than thesecond access point AP_(b) (supports MS_(a,b)), the first mobile stationMS_(c) associates itself with the second access point AP_(b).

It is understood that the parameters used to determine which accesspoint a particular mobile station should associate with can vary.Exemplary parameters include loading of the access point (number ofassociated stations), total traffic intensity through the access point,reports from individual stations, the measurement of the received signalpower from a subset of stations, mobile station priority weighting,access point priorities, and overall network loading.

It is further understood that the term “mobile station,” as used herein,should be construed broadly to include various wireless devices, such aslaptops, Personal Digital Assistants (PDAs), mobile phones, and thelike. Similarly, the term “access point” should be broadly construed toinclude transmitters/receivers in general that can provide a radio linkwith a mobile station.

Before describing further details of the present invention, some basicconcepts are now described. In conventional wireless networks havingmobile stations served by various access points, such as in a 802.11network, there is a standard procedure by which mobile stationsassociate themselves with an access point. The IEEE 802.11 standard isdefined in International Standard ISO/IEC 8802-111, “InformationTechnology-Telecommunications and Information Exchange Area Networks,”1999 Edition, which is hereby incorporated by reference in its entirety.Before a mobile station associates with an access point, it obtainsinformation of nearby access points by scanning the frequency channelsfor their beacons. The access points typically send out beacon framesperiodically.

In traditional WLANs, such as 802.11 networks, beacon powers of accesspoints are kept at a fixed level. The mobile station simply chooses theaccess point with the best (highest) signal strength for association.However, it is possible that one access point may be already overloaded,although it has the strongest signal strength to the mobile station.

While the following descriptions are applicable to 802.11 WLANs, it isunderstood that the invention applies to wireless networks in generalusing similar formats and mechanisms. Table 1 below shows the beaconframe body of a management frame of subtype Beacon with each of thelisted components specified in the 802.11 standard.

TABLE 1 Beacon frame body Order Information Notes 1 TimestampTime/day/etc information 2 Beacon Time between beacon intervals interval3 Capability Resource parameters, polling parameters, etc. information 4SSID Service Set ID (Wireless Network Name) 5 Supported Data Ratessupported by the system/device rates 6 FH The FH Parameter Setinformation element is Parameter present within Beacon frames generatedby STAs Set (Mobile STAtions) using frequency-hopping (FH) PHYs(PHYsical layer modulations) 7 DS The DS Parameter Set informationelement is Parameter present within Beacon frames generated by STAs Setusing direct sequence (DS) PHYs 8 CF The CF (Contention Free) ParameterSet Parameter information element is only present within Beacon Setframes generated by APs (Access Points) supporting a PCF (PointCoordination Function) 9 IBSS The IBSS (Independent Basic Service Set)Parameter Parameter Set information element is only present Set withinBeacon frames generated by STAs in an IBSS 10 TIM The TIM (TrafficInformation Map) information element is only present within Beaconframes generated by APs

Typically, the mobile station can operate in either Passive Scanningmode or Active Scanning mode. In Passive Scanning mode, the mobilestation listens to each channel scanned for no longer than a maximumduration defined by the ChannelTime parameter. The Active Scanning modeinvolves the generation of a Probe Request frame by the mobile stations,which is shown in Table 2 below, and the subsequent processing of areceived Probe Response frame, which is shown in Table 3, by the accesspoint.

TABLE 2 Probe Request frame body Order Information 1 SSID 2 Supportedrates

TABLE 3 Probe Response frame body Order Information Notes 1 TimestampTime/day/etc. information 2 Beacon interval Time between beaconintervals 3 Capability Resource parameters, polling parameters, etc.information 4 SSID Service Set ID (Wireless Network Name) 5 Supportedrates Data Rates supported by the system/device 6 FH Parameter Set TheFH Parameter Set information element is present within Beacon framesgenerated by STAs using frequency-hopping PHYs 7 DS Parameter Set The DSParameter Set information element is present within Beacon framesgenerated by STAs using direct sequency PHYs 8 CF Parameter Set The CFParameter Set information element is only present within Beacon framesgenerated by APs supporting a PCF 9 IBSS Parameter The IBSS ParameterSet information element is Set only present within Beacon framesgenerated by STAs in an IBSS

Conventionally, after scanning the access point beacons, the mobilestation adopts the BSSID (Basic Service Set ID: Access Point ID) andchannel synchronization information in a Beacon (passive) or ProbeResponse (active) coming from the access point with the best signalstrength. An Association/Reassociation Request is then issued by themobile station as it attempts to associate with the selected accesspoint. The access point then responds with an Association Response. Thecorresponding Association Request and Association Response frame formatsare shown below in Table 4 and Table 5, respectively. It is understoodthat further actions, such as authentication, take place before or afterthe association phase.

TABLE 4 Association/Reassociation Request frame body Order Information 1Capability information 2 Listen interval 3 SSID 4 Supported rates

TABLE 5 Association Response frame body Order Information 1 Capabilityinformation 2 Status code 3 Association ID (AID) 4 Supported rates

In accordance with the present invention, the mobile station selects anaccess point based upon beacon signal strength and access point loadinglevels.

FIG. 2 shows an exemplary access point 200 having a mobile stationmemory database 202 and a processor 204 for controlling the overalloperation of the access point. The database 202 can contain variousloading information including loading level, which can be provided asthe number of currently associated mobile stations 202 a, the linkquality 202 b, and the like. Further such information, such as mobilestation priority, the number of access points that a given mobilestation can “hear”, etc., will be readily apparent to one of ordinaryskill in the art.

The access point 200 includes a module 206 for assessing the medium loadcondition. More particularly, the module can determine the number ofmobile stations associated with the access point, their bandwidth usage,transmission queue length, number of error packets compared to the totaltraffic, and the like. This information can be used to determine aloading level for the access point 200, which can be used in accesspoint selection, as described further below.

The access point 200 can also include conventional components, such as awireless interface 250 having one or more RF transceivers, a networkinterface 252 for interacting with a wired network, and an I/O interface254 for communicating with various components, such as peripheralequipment.

In an exemplary embodiment, frequency option information can be conveyedin the Association/Reassociation request frame, which is shown above inTables 45. These two management frames contain the same CapabilityInformation field, which is used to indicate requested or advertisedcapabilities. In an illustrative embodiment, the length of theCapability Information field is two octets.

FIG. 3 shows an exemplary Capability Information field 300 having subfields of ESS 300 a, IBSS 300 b, CFPollable 302 c, CFPoll Request 302 d,and Privacy 302 e, together using five of the sixteen total bits. In anexemplary embodiment, three bits form a further subfield APLI 300 findicating the loading level of the access point. It is understood thatthe loading level can be presented in a variety of formats including rawnumber of associated mobile stations, discrete values indicative ofloading level, percent of rated capacity, and the like. The remainingbits 300 g of the Capability Information field 300 are reserved.

FIG. 4 shows an exemplary mobile station 400 having load-based accesspoint selection in accordance with the present invention. The mobilestation 400 includes a processor 402 for controlling the overall deviceoperation and a memory 404 for storing various information, such asconfiguration parameters 404 a, access point IDs 404 b, access pointbeacon signal power levels 404 c, and access point loading levelinformation 404 d. The mobile station 400 can further include aconventional wireless interface 450 for transmitting and receiving radiofrequency signals, and an I/O interface 452.

The mobile station 400 further includes an association module 406 forselecting an access point for association based upon the detected accesspoint beacon signal power levels and access point loading. It isunderstood that the association module can select access points using avariety of functions using a range of parameters.

For example, assume that the beacons of access points AP₁, AP₂, . . .AP_(N) can be received by a given mobile station. For each access pointAP, through its beacon signal, the mobile station can obtain variousparameters, such as signal strength (RSSI), number of associatedstations, traffic intensity and other related parameters. Theseparameters can be referred to as the signature vector V_(i) for theaccess point AP_(i). A cost function f(V_(i)) can be defined tocalculate the cost if the mobile station associates with a given accesspoint AP_(i). Once the cost function is defined as desired, theassociation module of the mobile station chooses the access point APwith the minimal cost of association.

In one particular embodiment, the association module uses the number ofstations associated with the AP_(s) as the sole parameter of the costfunction. Let n₁, n₂, . . . n_(N) be the number of stations associatedwith AP₁, AP₂, . . . AP_(N), and let f(V_(i))=x_(i). Then theassociation module of the mobile station chooses the access point APwith the fewest number of associated stations for association, i.e.,AP_selected={AP_(j)|n_(j)=min{n₁, n₂, . . . n_(N)}}, which is known asthe Join-the-shortest-queue (JSQ) algorithm.

This algorithm provides a mechanism for the mobile stations to choosethe best server available and enables the mobile stations to explore thespatial diversity of a distributed network.

In another embodiment, it is assumed that there is an acceptable levelof beacon signal power level P_(a). When a mobile station receivesbeacon signals from multiple access points, each of which has a powerlevel equal or greater than P_(a), the access point with the lowesttraffic loading among such plurality of access points is selected forassociation with the mobile station.

In one embodiment, the access point assesses the medium condition andinserts loading information in the Capability Information field (FIG. 3)of a Beacon/Probe Response frame. While in the scanning mode, the mobilestation receives and records the access point loading informationdelivered in the Beacon/Probe Response frame. The mobile station canthen evaluate the information and select an access point based uponbeacon signal strength and access point loading levels, as describedabove.

One skilled in the art will appreciate further features and advantagesof the invention based on the above-described embodiments. Accordingly,the invention is not to be limited by what has been particularly shownand described, except as indicated by the appended claims. Allpublications and references cited herein are expressly incorporatedherein by reference in their entirety.

1-17. (canceled)
 18. A method of facilitating distribution of one ormore access points in a wireless network comprising: obtainingparameters, said parameters comprising load level information of anaccess point, access point priority, and overall network loading, saidload level information comprising a transmission queue length and anumber of transmitted error packets; storing said parameters; andtransmitting said parameters to at least one mobile station.
 19. Themethod of claim 18 further comprising: transmitting a beacon signal fromthe access point to the at least one mobile station.
 20. The method ofclaim 18 wherein transmitting said parameters comprises: transmittingsaid parameters in a beacon frame.
 21. The method of claim 18 furthercomprising: receiving a request for parameters from the at least onemobile station, wherein transmitting said parameters to the at least onemobile station is in response to said received request.
 22. The methodof claim 18 further comprising: storing link quality information. 23.The method of claim 18 further comprising: connecting the access pointto a plurality of access points to form an 802.11 network.
 24. Themethod of claim 18 further comprising: associating the access point withthe at least one mobile station.
 25. The method of claim 18 furthercomprising: determining a cost function based on said parameters. 26.The method of claim 25 further comprising: transmitting said costfunction to the at least one mobile station.
 27. The method of claim 25wherein determining the cost function further comprises determining thecost function based on a traffic intensity of the access point.
 28. Themethod of claim 25 wherein determining the cost function furthercomprises determining the cost function based on a signal strength ofthe access point.